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Linux System Startup By Rick Mann

Linux System Startup

The geek word for starting up a computer is bootstrapping. The short version is booting or boot. The initial part of this process is performed by code stored in ROM. This is code that is general in nature rather than being specific to Linux. Its task is to load the Linux-specific loader and turn control over to it.

Boot Loaders

The boot loader is the program loaded by the ROM (either the BIOS on the motherboard or device-specific code like on a SCSI disk controller board). There are two popular boot loaders for PCs. LILO is the traditional loader and GRUB is the newer one. Each program has the task of grabbing some configuration information, loading the Linux (or other) kernel and turning over control.

The most significant difference between LILO and GRUB is how it gets the configuration information. The configuration for LILO is saved in a static form by running the lilo command. This information is written to either the master boot record (MBR) of the disk or to the boot record of the Linux root partition. The configuration information used by the lilo command is normally stored in /etc/lilo.conf. Here is a sample configuration file.

boot=/dev/hda   # boot loader to MBR
root=/dev/hda1  # root partition
install=/boot/boot.b
map=/boot/map
delay=50        # 5 second delay before auto-boot
image=/vmlinuz  # kernel
  label=linux   # name to refer to entry
  read-only
image=/vmlinuz.old      # backup entry
  label=old
  read-only

In this example, there are two possible kernels to boot: /vmlinuz and /vmlinuz.old. At the LILO prompt you can select between them by entering linux to select the current one or old to select the backup one. Pressing the TAB key at the LILO prompt will list these options. If you rebuild your kernel or want to make any other change you will need to rerun the lilo command to re-read the configuration file and re-install LILO with this new configuration information.

GRUB reads the configuration file at boot time. The MBR is only 512 bytes. The portion of GRUB that is installed in the MBR does some basic initialization of the system, figures out how to access the boot drive and then loads the rest of GRUB from the drive.

GRUB is installed by the grub-install program. There should be a man or info page available with the details. The grub info page is also very helpful. The configuration file is generally located in the /boot/grub directory. SuSE puts it in menu.lst and Red Hat in grub.conf. Here is a sample configuration file:

default 0
timeout 8
gfxmenu (hd0,1)/boot/message

title Linux
    kernel (hd0,1)/boot/vmlinuz root=/dev/hda2 desktop showopts
    initrd (hd0,1)/boot/initrd

title Failsafe
    kernel (hd0,1)/boot/vmlinuz root=/dev/hda2 showopts ide=nodma apm=off acpi=o
ff vga=normal nosmp noapic maxcpus=0 3
    initrd (hd0,1)/boot/initrd

title Memory Test
    kernel (hd0,1)/boot/memtest.bin

If you are sharing the computer with a proprietary OS from Redmond, take note that those people don't realize there are other operating systems available. That means that when you install their system the just overwrite the MBR. If you install their software first followed by Linux, all should be okay and you will be able to boot either OS.

Run Levels

Run levels offer you an array of system configurations. Unless told otherwise, the system will come up at the default run level which is typically level 3 or level 5. You can alter this behavior by entering the label name in LILO or the word boot in GRUB followed by the word single at the boot loader prompt.

There are seven standard run levels: 0 through 6. Level 0 means shutdown, level 1 is single-user mode and level 6 means reboot. The other levels are available at your discretion to set up various system configurations. The most typical is to use run level 3 as a fully-operational system without the GUI (X) running and level 5 like level 3 with the GUI. On many systems, there is a run level called S which is like run level 1 but requires the root password to be entered. This is there for security reasons.

The contents of the file /etc/inittab determine what action is to be taken at each run level and also specifies the default run level. Here is a sample of what might appear in /etc/inittab:

#
# /etc/inittab
#
# This is the main configuration file of /sbin/init, which
# is executed by the kernel on startup.
#

# The default runlevel
id:5:initdefault:

# /etc/init.d/rc takes care of runlevel handling
#
# runlevel 0  is  System halt   (Do not use this for initdefault!)
# runlevel 1  is  Single user mode
# runlevel 2  is  Local multiuser without remote network (e.g. NFS)
# runlevel 3  is  Full multiuser with network
# runlevel 4  is  Not used
# runlevel 5  is  Full multiuser with network and xdm
# runlevel 6  is  System reboot
#
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6

# what to do in single-user mode
ls:S:wait:/etc/init.d/rc S
~~:S:respawn:/sbin/sulogin

# what to do when CTRL-ALT-DEL is pressed
ca::ctrlaltdel:/sbin/shutdown -r -t 4 now

# getty-programs for the normal runlevels
# :::
# The "id" field  MUST be the same as the last
# characters of the device (after "tty").
1:2345:respawn:/sbin/mingetty --noclear tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6

The line id:5:initdefault: sets the default run level to 5. The lines of this form l1:1:wait:/etc/init.d/rc 1 invoke the script /etc/init.d/rc passing it the run level as an argument. This script then starts the processes associated with the specific run level (and stops other processes). All of the scripts to control each process are also located in the /etc/init.d directory.

Typically, which processes are to be started and stopped at each run level are located in sub-directories (for example, rc5.d for run level 5) of /etc/init.d. In each of these runlevel-specific directories, symbolic links are used to identify the processes. Link names starting with K refer to processes that are to be stopped (killed) and link names starting with S refer to those which are to be started. The links are accessed alphabetically which means the kill scripts are run first and the order of the scripts within the kill and start lists are controlled by using a 2-digit number immediately following the K or S.

I said typically as this is the standard way to handle this information. Some vendors use slightly different schemes but, in all cases, the generainit program are what controls the whole process. If you are familiar with how UNIX handles startup, this is very similar to System V Init.

If there were no problems encountered along the way your system should now be at the chosen run level. Once the system is up and running you can change run levels by logging on as root and using the init command. For example, to change to run level 3, you would enter init 3.

Rick Mann has been programming in C and working with POSIX-compliant operating systems for 12 years.

None provided.